...
In order to use a row select function in a template, you must include the "SelectRowNumber" widget.
To use the widget, enter the IF block- IF{MERGEFIELD RowNumber}= "{MERGEFIELD SelectedRowNumber}". Then
After you have done this, you can then construct your template as usual, using mergefields from your customer custom field table.
When Creating the Document, Select the Row you Want to be Populated
...